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Zircon Halogen Geochemistry: Insights Into Hadean-Archean Fluids

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Zircon Halogen Geochemistry: Insights Into Hadean-Archean Fluids © 2019 The Authors Published by the European Association of Geochemistry Zircon halogen geochemistry: Insights into Hadean-Archean fluids H. Tang1*, D. Trail1,2, E.A. Bell1, T.M. Harrison1 Abstract doi: 10.7185/geochemlet.1905 Understanding the distribution of halogens in rocks can potentially trace ancient lithosphere, hydrosphere, and atmosphere interactions. Although no sedimentary rocks older than 3.8 Ga are known, insights into sediment-atmosphere exchange on early Earth could be obtained from knowledge of halogen contents in ancient zircons. Here we present the first study of halogen abundances in Jack Hills zircons together with younger zircons of known provenance to provide geologic context. The relatively low (ca. 0.1-0.6 μg/g) chlorine concentrations in most Hadean and Archean Jack Hills zircons are similar to the average concentration in younger igneous zircons. In contrast, significant Cl enrichments are found in a subset of ca. 3.9–3.8 Ga zircons ([Cl]average= 1.43 ± 0.27 μg/g) that appear to record halogen transport under hydrothermal conditions. Such Cl-bearing fluids in early Earth history may reflect extraction of halogens from the interior to near surface environments. Received 21 September 2018 | Accepted 20 January 2019 | Published 19 February 2019 Introduction with magmatic Th/U and average Ti-in-zircon temperatures ca. 680 °C. Group II is a distinctive subset of the zircons with Earliest Earth remains one of the great frontiers for funda- ages 3.9–3.8 Ga that show distinctive chemistry relative to mental discovery as little is known about key events such as Group I (lower Th/U, P, and Ce; higher U and Hf). Their Ti-in- the mechanisms and timing of life’s origin. Given that >3.8 Ga zircon temperatures are mostly subsolidus, and these zircons rocks are exceedingly rare and no rock older than 4.03 Ga are either dark and homogeneous in cathodoluminescence is known (cf. O’Neil et al., 2008), constraining conditions on or display patchy zoning. The Group II characteristics imply early Earth has largely relied on the only materials identified recrystallisation during thermal events (Hoskin and Black, from the Hadean eon: ~4.4 to 3.8 Ga detrital zircons (Mojzsis 2000). Such episodes may have provided a mechanism for et al., 2001; Peck et al., 2001; Harrison et al., 2008; Holden et al., volatile transport in early fluids. Given the significance of 2009; Bell et al., 2011). Oxygen isotopes, rare earth element halogens in lithosphere-hydrosphere- atmosphere interactions, (REE) patterns, crystallisation temperatures, and inclusions we focus here on their abundances in Jack Hills zircons and in detrital zircons from the Jack Hills, Western Australia, are examine fractionations between nominally mobile (Cl) and suggestive of the presence of recycled crustal material that had immobile (F) halogens in a variety of reservoirs (e.g., magmatic interacted with liquid water at low temperature (e.g., Watson and metamorphic systems). and Harrison, 2005; Trail et al., 2011; Bell and Harrison 2013; Harrison et al., 2017) and show how experimental calibra- tions can permit zircon to act as an environmental monitor Samples and Methods of lithosphere and surface processes. On the other hand, the lack of a known sedimentary record older than 3.83 Ga Our Jack Hills zircon suite ranges in U-Pb age from 4.2 to (Manning et al., 2006) limits our understanding of the vola- 3.4 Ga (Table S-2). Oxygen isotope ratios and REE patterns tile evolution on earliest Earth. To address this issue, one in some of these zircons have been investigated previously approach is to trace those volatile components that partition and shown to lack secondary alteration (Table S-2, S-3, and at measureable levels into zircon. Previous investigations of S-4, Fig. S-7 and S-8; Bell et al., 2011, 2014, 2016; Bell and oxygen isotopes and trace elements in Jack Hills zircons (e.g., Harrison, 2013). Given the possible felsic parent sources of Jack Mojzsis et al., 2001; Cavosie et al., 2005; Trail et al., 2007; Bell Hills zircon (Bell et al., 2018), zircons from igneous (I-type) et al., 2011) indicate two distinctive protolith sources: Group and sedimentary (S-type) granitoids from the Phanerozoic I and Group II (Bell and Harrison, 2013). Group I is similar Lachlan Fold Belt (LFB), Australia, and Mesozoic batho- in most respects to >3.9 Ga and <3.8 Ga Jack Hills zircons, liths of southern California, USA, were also investigated as 1. Department of Earth, Planetary and Space Sciences, University of California, Los Angeles, Los Angeles CA, 90095 USA 2. Department of Earth and Environmental Sciences, University of Rochester, Rochester, NY, 14618 USA * Corresponding author (email: [email protected]) Geochem. Persp. Let. (2019) 9, 49-53 | doi: 10.7185/geochemlet.1905 49 Geochemical Perspectives Letters Letter analogues for establishing geologic provenance. Detection is consistent with the prevailing populations for their corre- limits ([F] = 0.78 ± 0.07 μg/g and [Cl] = 0.12 ± 0.02 μg/g with sponding time periods (Cavosie et al., 2005; Trail et al., 2007; 2 se.; Fig. S-4) were determined by analysing synthetic zircons Bell et al., 2016) and no obvious correlation between δ18O and grown in halogen-free conditions by the 1 atm flux method Cl concentration is observed. The prominent excursion of Cl (e.g., Trail et al., 2016). We also synthesised zircon grains in a content in multiple 3.9–3.8 Ga zircons is higher than that seen halogen-rich fluid to explore Cl (and F) partitioning in zircon thus far in post-Archean grains. and to produce an analogue of recrystallised zircon (see Supplementary Information for experimental details). The in situ analyses of Cl and F in zircon were performed using the CAMECA ims1290 ion microprobe. Analytical details are described in the Supplementary Information. Mud Tank zircons implanted with known F or Cl isotopic dosages were analysed as concentration standards. Cracks/inclusions in samples were identifiedvia secondary and backscattered electron SEM imaging. Analysis zones were limited to regions without cracks or inclusions. Results Young zircons (1.1–0.1 Ga) display uniform F concentrations (Fig. 1) within uncertainty (1.80 ± 0.11 μg/g), except AS3 zircon (Duluth Complex, Minnesota), which we attribute to enrich- ment during hydrothermal alteration (Takehara et al., 2018). All the post-Archean zircons in this study exhibit system- atically low Cl contents, some of which cannot be resolved from the baseline. The average concentration is determined as 0.31 ± 0.04 μg/g. Fluorine concentrations in all 58 Jack Hills samples are slightly higher than those in zircons from the LFB and southern Californian granitoids (2.28 ± 0.19 μg/g). Chlorine concentrations and Cl/F ratios in Jack Hills zircons appear to vary with age (Fig. 1). Significantly high Cl concentration ([Cl]average = 1.19 ± 0.32 μg/g) is observed in a subset of zircons within a narrow age range of 3.9–3.8 Ga. Archean grains with ages from 3.8 to 3.4 Ga display Cl contents identical to the young zircons ([Cl]average = 0.34 ± 0.04 μg/g), pointing to a magmatic environment with halogens levels roughly similar to modern igneous systems. Two Jack Hills zircons older than 3.9 Ga yield exceptionally high Cl contents (RSES 178-7.14 and RSES 178-8.8 with Pb-Pb ages of 4012 Ma and 4073 Ma, respectively), although the halogen measurements on the other 25 grains are within error of the range of 0.1-0.6 μg/g in the young zircons. Halogens in Jack Hills Zircons: Primary or Secondary? Given that mobile elements can be substituted into the zircon lattice during secondary alteration (e.g., Bell et al., 2016), care must be taken to ascertain that Cl and F concentrations reported for Jack Hills zircons do not represent post- crystallisation hydrothermal alteration. In our study, U-Pb ages in all Cl-rich zircons are concordant within ±10 % (Bell and Harrison, 2013) indicating little Pb loss or U gain after 3.8 Ga. In addition, two indices, including (Sm/La)N (Hoskin, 2005) and the LREE-I [= (Dy/Nd) + (Dy/Sm); Bell et al., 2016], can test for possible alteration effects leading to crystalline degeneration. The high values of both indices for all grains (except one Archean zircon) are indicative of crystalline zircon, including the Cl-rich grains Figure 1 Fluorine concentrations in Jack Hills zircons ([F] (see Bell et al., 2016) (Fig. 2). The halogen abundances are inter- average = 2.28 ± 0.19 μg/g) are generally indistinguishable from those in young granitic zircons ([F]average = 1.80 ± 0.11 μg/g). preted as reflecting concentrations present during crystallisa- Elevated Cl concentrations observed in eight 3.9–3.8 Ga zircons tion from their primary sources magmas. The distribution of (black columns) yield distinctly high Cl/F ratios (Cl/F > 0.3) when 18 oxygen isotopes in our samples (δ Oave= 5.87 ± 0.17; Fig. S-7) compared with other Jack Hills zircons. Geochem. Persp. Let. (2019) 9, 49-53 | doi: 10.7185/geochemlet.1905 50 Geochemical Perspectives Letters Letter Figure 2 High values of (Sm/La)N and LREE-I [(Dy/Nd) + (Dy/Sm)] in Jack Hills sample grains imply that most of samples, especially Cl-rich zircons, are primary. The elevated Cl concentrations in zircons therefore derived from primary sources. Group II Zircons: Recrystallisation °C, indistinguishable from the Hadean distribution (Watson in Cl-rich Fluids? and Harrison, 2005). A possible mechanism to interpret the low Ti contents Bell and Harrison (2013) classified 3.9–3.8 Ga Jack Hills and other characteristics of Group II zircons is metamorphic zircon grains into two categories.
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